Method and apparatus for monitoring operation of a pilot-controlled pressure relief valve

ABSTRACT

A method for determining effective area coefficient for a pilot operated safety relief valve. The relief valve may have a piston with an upper surface area, an inlet, and a dome. The method may include determining a total force acting on the piston (F total ) and determining a downward force (F dome ) on the piston due to dome pressure. The method may further include determining an upward force on the piston due to inlet pressure (F main ) by subtracting the downward force (F dome ) from the total force (F total ) and determining an instantaneous Effective Area coefficient (A e ) by dividing the upward force on the piston (F main ) by a main inlet pressure (P main ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to the field of testingpilot-controlled safety relief valves, and more particularly to thefield of measuring instantaneous/dynamic effective area coefficient, andeffective area vs. lift function, of pilot-controlled pressure reliefvalves.

2. Discussion of Related Art

In general, pilot controlled Safety Relief Valves (SRVs), have a mainvalve composed of a piston with a dome volume behind it, and a pilotvalve for filling/dumping the dome volume. The main valve piston isexposed to pipe inlet pressure below and dome pressure above. Thedifference in exposed surface areas between the top and bottom of thepiston keep the main valve closed, until the pilot valve dumps the gasin the dome volume, which lowers the dome pressure and causes the pistonto lift.

As the main valve piston lifts and begins to relieve pressure from theprotected system, the inlet pressure may only push against a portion ofthe exposed piston surface area as a result of gas flow anddynamic/parasitic effects. That portion coefficient is known as the“Effective Area” coefficient. In a steady flowing or slowly movingvalve, this coefficient depends on piston lift. But in a rapidly movingvalve, the Effective Area coefficient depends strongly on pistonvelocity, gas inertia, gas compliance and more. Since the analysis ofvalve instabilities involves rapidly moving SRVs, dynamic/parasiticeffects such as piston inertia and gas inertia/compliance cannot beignored.

One current method for measuring the Effective Area coefficient of pilotcontrolled SRVs is as follows: (1) raise the piping system, leading tothe main valve inlet, up to an operating pressure; (2) keep the valveopened at different piston lift points, which is often done by holdingthe valve piston with a screw; (3) at each lift point, measure the liftforce on the valve piston, which is often done with a load cell placedbehind the valve piston; and (4) divide the lift force by the operatinginlet pressure to obtain the coefficient.

There are variations of this method, but they all require steady-stateor quasi-steady flow conditions. As a result, when dealing with unstablevalves or rapidly moving valves, these methods fail because they do notconsider valve dynamic/parasitic effects such as piston inertia, gasinertia, gas compliance and more, as previously noted.

Current methods do not take dynamic effects into account in themeasurement/calculation of Effective Area coefficient. Certifications ofvalves require manufacturers to analyze valves in steady-state flowingconditions. The common belief is that valve stability/performanceproblems depend exclusively on fixed parameters such as pipe lengths andpipe turns/intersections. For the reasons previously noted, suchtechniques may result in inaccurate values of the Effective Areacoefficient for an SRV.

Thus, there is a need for an improved method for measuring theinstantaneous/dynamic Effective Area coefficient and “Effective Area vs.Lift” function of pilot-controlled SRV's.

SUMMARY OF THE INVENTION

The disclosed method is an improved technique for measuring theinstantaneous Effective Area coefficient and Effective Area vs. Liftfunction of rapidly moving pilot-controlled SRVs.

A method is disclosed for determining effective area coefficient for apilot operated safety relief valve, the relief valve having a pistonwith an upper surface area, an inlet, and a dome. The method comprisesthe steps of: determining piston velocity (P_(vel)) and pistonacceleration (P_(acc)); determining a total force acting on the piston(Ftotal) based on a mass of the piston and the piston acceleration;determining a downward force on the piston due to dome pressure(F_(dome)) by multiplying the dome pressure (P_(dome)) with the pistonupper surface area (A_(UpperSurfaceArea)); determining an upward forceon the piston due to inlet pressure (F_(main)) by subtracting thedownward force from the total force (F_(total)); determining a lift ofthe piston (P_(lift)); determining an instantaneous Effective Areacoefficient (A_(e)) by dividing the upward force on the piston(F_(main)) by a main inlet pressure (P_(main)); and plotting theEffective Area coefficient vs. P_(lift) to determine the Effective Areacoefficient (A_(e)) vs. piston (P_(lift)) function.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing illustrates an exemplary embodiments of thedisclosed device so far devised for the practical application of theprinciples thereof, and in which:

FIG. 1 is an exemplary safety relief valve;

FIG. 2 is an exemplary arrangement for performing the disclosed method;

FIG. 3 shows two plots of the Effective Area Coefficient vs. Lift of avalve run with different piston seat retainers;

FIG. 4 shows plots of Effective Area vs. Lift function for two differentunstable runs on a valve with a flat nose retainer

DESCRIPTION OF EMBODIMENTS

The disclosed method can be used to measure the instantaneous/dynamic“Effective Area Coefficient” and “Effective Area vs. Lift” function ofpilot-controlled pressure relief valves. The disclosed arrangement canbe used to obtain many other dynamic properties of valves, such aspiston velocity and acceleration, kinetic and potential energy,frictional losses and much more. In one embodiment, the disclosed methodcalculates the instantaneous Effective Area coefficient of apilot-controlled SRV using field sensor data.

In general, pilot controlled SRVs (Safety Relief Valves), as shown inFIG. 1, have a main valve 1 comprising an inlet port 2, an outlet port4, a piston 6 having a first piston face 8 exposed to main inletpressure (i.e., the pressure of the system being protected), and asecond piston face 10 exposed to dome pressure associated with a domevolume 12. The piston 6 is shown having a stop bolt 14 for limitingpiston lift.

A pilot valve (not shown) is in communication with the dome volume 12for filling/dumping the dome volume. As previously noted, the piston 6is exposed to pipe inlet pressure below it (via the valve inlet port 2),and dome pressure above it. The difference in exposed surface areasbetween the faces 8, 10 of the piston 6 keep the valve 1 closed, untilthe pilot valve dumps the gas in the dome volume 12, lowering the domepressure, and causing the piston 6 to move upward, opening a pathbetween the inlet port 2 and the outlet port 4.

As the main valve piston lifts and starts relieving the protectedsystem, gas flows around the piston. The gas applies a pressure-dragforce that pushes the piston upwards. The inlet pressure, however, onlyacts against a portion of the exposed piston surface area. That portionis known as the “Effective Area” coefficient. Effective Area is the areawhich, when multiplied by the inlet pressure, equals to the upwardpressure-drag force due to the gas flow. In a steady flowing or slowlymoving valve, the Effective Area coefficient depends on piston lift. Butin a rapidly moving valve, the Effective Area Coefficient dependsstrongly on piston velocity, gas inertia, gas compliance, frictionallosses and more.

Referring to FIG. 2, the disclosed test arrangement includes a pressuresensor 16 positioned in the valve's dome 12 for measuring dome pressure,a pressure sensor 18 integral to the valve 1 to measure inlet pressure,and an inductive sensor such as a linear variable differentialtransformer (LVDT) lift sensor 20 to measure valve piston 6 lift. Thesetup as shown in FIG. 2, is simple and, as discussed, uses only threesensors. The illustrated arrangement uses an Anderson-Greenwood (A-G)853 series P-orifice valve with an 800 series pilot modified to work atlower pressures.

The disclosed arrangement is unique in that it enables calculation ofthe dynamic Effective Area coefficient, as opposed to standard methodswhich are based on steady state flows. It also allows for on-linecalculation of instantaneous Effective Area coefficient. This can, inturn, be used for on-line analysis of valve performance, valve stabilityand much more. The FIG. 2 arrangement enables real-time data to beobtained from the sensors 16, 18, 20, which provide a direct measure ofdome pressure (P_(d)), inlet pressure (P_(main)), and piston lift(P_(lift)). Using these values, and knowing the piston mass (P_(mass)),the following analysis steps provide a real time determination/plot ofthe instantaneous Effective Area coefficient (A_(e)):

1. Calculate Piston Velocity (P_(vel)) and Piston Acceleration (P_(acc))by differentiating the piston lift signal twice:

${a.\mspace{14mu} P_{vel}} = \frac{P_{lift}}{t}$${b.\mspace{14mu} P_{acc}} = \frac{P_{vel}}{t}$

2. Calculate the total force acting on the valve's piston (F_(total)) byusing Newton's second law:

F _(total) =P _(mass) *P _(acc)

3. Calculate the downward force on the valve's piston due to domepressure (F_(dome)) by multiplying the dome pressure (P_(dome)) with thepiston upper surface area (A_(UpperSurfaceArea)):

F _(dome) =P _(dome) *A _(UpperSurfaceArea)

4. Calculate the upward force on the valve's piston due to inletpressure (F_(main)) by subtracting the dome force (F_(dome)) from thetotal force (F_(total)):

F _(main) =F _(total) −F _(dome)

5. Calculate the instantaneous Effective Area coefficient (A_(e)) bydividing the upward force on the valve's piston (F_(main)) by the maininlet pressure (P_(main)):

$A_{e} = \frac{F_{main}}{P_{main}}$

6. Calculate the Effective Area coefficient (A_(e)) vs. Lift (P_(lift))function by plotting the Effective Area coefficient vs. piston lift:

Plot A _(edefi A) _(e)(P _(lift))

FIG. 3 shows plots generated using the disclosed method applied to aslowly moving valve, run with two different piston seat retainers.Specifically, FIG. 3 shows two plots of the Effective Area Coefficient(meters²) vs. Lift (meters) of a valve run with different piston seatretainers. This plot shows the real-time generated A_(e)(P_(lift))curves 22, 24 for a quasi-steady valve with different piston seatretainers. Curve 22 is representative of a valve configuration usingstandard flat nose seat retainer, while curve 24 is representative of avalve configuration using 40-degree cone seat retainer. Even in thisquasi-steady valve (an ideal case), the plot shows system hysteresis dueto parasitic effects.

The ability of this method to generate real-time plots is advantageouswhen applied to rapidly opening/closing valves, as is the case forunstable valves. In such cases, parasitic effects can create highlynon-linear interfaces between the piston and the flowing gas, which varywildly from the smooth linear steady state condition.

FIG. 4 shows a plot of the Effective Area vs. Lift function for twounstable runs on a valve using the test arrangement of FIG. 2. This plotshows the real-time generated A_(e)(P_(lift)) curve for a rapidly movingunstable valve with a flat nose retainer. In this case the non-linearityof the piston-gas interface are brought to the surface when the valvegoes unstable. The plots show that when the valve becomes unstable, theeffective area function varies non-linearly from its steady state form.

The method described herein may be automated by, for example, tangiblyembodying a program of instructions upon a computer readable storagemedia capable of being read by machine capable of executing theinstructions. A general purpose computer is one example of such amachine. A non-limiting exemplary list of appropriate storage media wellknown in the art would include such devices as a readable or writeableCD, flash memory chips (e.g., thumb drives), various magnetic storagemedia, and the like.

The features of the system and method have been disclosed, and furthervariations will be apparent to persons skilled in the art. All suchvariations are considered to be within the scope of the appended claims.Reference should be made to the appended claims, rather than theforegoing specification, as indicating the true scope of the disclosedmethod.

The functions and process steps disclosed herein may be performedautomatically or wholly or partially in response to user command. Anactivity (including a step) performed automatically is performed inresponse to executable instruction or device operation without userdirect initiation of the activity.

The systems and processes of FIGS. 1-4 are not exclusive. Other systems,processes and menus may be derived in accordance with the principles ofthe invention to accomplish the same objectives. Although this inventionhas been described with reference to particular embodiments, it is to beunderstood that the embodiments and variations shown and describedherein are for illustration purposes only. Modifications to the currentdesign may be implemented by those skilled in the art, without departingfrom the scope of the invention. Further, any of the functions and stepsdescribed herein may be implemented in hardware, software or acombination of both and may reside on one or more processing deviceslocated at any location of a network linking the elements of the systemor another linked network, including the Internet.

Thus, although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

1. A method for determining an effective area coefficient for a pilotoperated safety relief valve, the relief valve having a piston with anupper surface area, an inlet, and a dome, the method comprising:determining a total force acting on the piston (F_(total)); determininga downward force (F_(dome)) on the piston due to dome pressure;determining an upward force on the piston due to inlet pressure(F_(main)) by subtracting the downward force (F_(dome)) from the totalforce (F_(total)); and determining an instantaneous effective areacoefficient (A_(e)) by dividing the upward force on the piston(F_(main)) by a main inlet pressure (P_(main)).
 2. The method of claim1, the determining the total force comprising: determining mass(P_(mass)) of the piston; determining acceleration (P_(acc)) of thepiston; and calculating the total force according toF_(total)=P_(mass)*P_(acc).
 3. The method of claim 2, the determiningP_(acc) comprising: determining piston lift (P_(lift)) at a plurality ofinstances in time t; differentiating P_(lift) as a function of time todetermine piston velocity P_(vel), wherein dP_(lift)/dt=P_(vell); anddifferentiating P_(vel) as a function of time to determine P_(acc),wherein dP_(vel)/dt=P_(acc).
 4. The method of claim 3, furthercomprising plotting A_(e) vs. P_(lift) for a plurality of piston liftpositions to determine an effective area coefficient vs. piston liftfunction.
 5. The method of claim 3, further comprising providing a liftsensor to measure P_(lift).
 6. The method of claim 5, the lift sensorcomprising a linear variable differential transformer lift sensor. 7.The method of claim 1, the determining F_(dome) comprising: measuringdome pressure (P_(dome)); determining an upper surface area(A_(UpperSurface)) of the piston; and multiplying P_(dome) byA_(UpperSurface).
 8. The method of claim 1, comprising: providing a domepressure sensor to measure P_(dome): and providing an inlet pressuresensor configured to measure P_(main).
 9. A relief valve monitoringsystem, the system arranged to monitor a pilot controlled safety reliefvalve that includes a piston having an upper surface area, an inletdisposed on a first side of the piston, and a dome disposed on a secondside of the piston adjacent the upper surface area, the systemcomprising: a dome pressure sensor configured to measure pressure of thedome; an inlet pressure sensor for measuring inlet pressure; and a liftsensor for measuring piston lift, wherein the dome pressure sensor,inlet pressure sensor and lift sensor are interoperable to determine aninstantaneous effective area coefficient (A_(e)) of the relief valveduring movement of the piston.
 10. The relief valve monitoring system ofclaim 9, wherein the system is configured to: determine a total forceacting on the piston (F_(total)); determine a downward force (F_(dome))on the piston due to the measured dome pressure; determine an upwardforce on the piston due to inlet pressure (F_(main)) by subtracting thedownward force (F_(dome)) from the total force (F_(total)); anddetermine the instantaneous effective area coefficient (A_(e)) bydividing the upward force on the piston (F_(main)) by the measured inletpressure (P_(main)).
 11. The relief valve monitoring system of claim 9,wherein the system is configured to: determine acceleration (P_(acc)) ofthe piston using the lift sensor; and calculate the total forceaccording to F_(total)=P_(mass)*P_(acc.), where P_(mass) is the mass ofthe piston.
 12. The relief valve monitoring system of claim 9, whereinthe system is configured to: measure piston lift (P_(lift)) using thelift sensor while the piston is in motion at a plurality of instances intime t; differentiate P_(lift) as a function of time to determine pistonvelocity P_(vel), wherein dP_(lift)/dt=P_(vell); and differentiateP_(vel) as a function of time to determine P_(acc), whereindP_(vel)/dt=P_(acc).
 13. The relief valve monitoring system of claim 9,wherein the system is configured to plot A_(e) vs. P₁ for a plurality ofpiston lift positions to determine an effective area coefficient vs.piston lift function.
 14. The relief valve monitoring system of claim 9,the lift sensor comprising a linear variable differential transformerlift sensor.
 15. The relief valve monitoring system of claim 9, whereinthe system is configured to determine A_(e) vs P_(lift) when the pistonis traveling in a first direction and in a second direction opposite thefirst direction.
 16. The relief valve monitoring system of claim 15,wherein the system is configured to determine hysteresis in an A_(e) vsP_(lift) function between a first set of values of A_(e) obtained for afirst set of P_(lift) positions when the piston is traveling in thefirst direction and a second set of values of A_(e) obtained for thefirst set of P_(lift) positions when the piston is traveling in thesecond direction.
 17. The relief valve monitoring system of claim 15,wherein the system is configured to detect valve instability bydetermining a non-linearity in an A_(e) vs P_(lift) function.
 18. Amethod for dynamically determining effective area coefficient for apilot operated safety relief valve, comprising: calculating, using liftposition measurements of a piston of the relief valve, a total forceacting on the piston (F_(total)) during operation of the piston;measuring, during operation of the piston, a downward force (F_(dome))on the piston due to dome pressure of a dome disposed on a first side ofthe piston; measuring, during operation of the piston, a main inletpressure (P_(main)) of an inlet disposed on a second side of the piston,the second side being opposite the first side of the piston; anddetermining an instantaneous effective area coefficient (A_(e)) bydividing an upward force on the piston due to main inlet pressure(F_(main)) by the main inlet pressure P_(main), whereinF_(main)=F_(dome)−F_(total).
 19. The method of claim 1, the calculatingthe total force comprising: determining piston lift (P_(lift)) at aplurality of instances in time t; doubly differentiating P_(lift) as afunction of time to determine piston acceleration P_(acc); andcalculating total force by F_(total)=P_(mass)*P_(acc), wherein P_(mass)is mass of the piston.
 20. The method of claim 19, further comprisingplotting A_(e) vs. P₁ for a plurality of piston lift positions todetermine an effective area coefficient vs. piston lift function.